Hỏi gọi đối tượng ra khỏi lớp trong private

#include <iostream>
#include <iomanip>
#include <string.h>
using namespace std;
class sinhvien
{ private:
    char ten[20];
	float diemtoan;
	float diemly;
	float diemhoa;
	float dtb;
	char mssv[20];
 public:
 	
 	void nhap();
 	void xuat();
 	string  hebon(float dtb);
 	char tensv();

};
char sinhvien::tensv()
{   

	}
void sinhvien::nhap()

{
sinhvien sv;
cout<<" nhap ten sv :";
    cin.ignore(1);
    cin.get(ten,20);
cout<<"nhap MSSV     :";
    cin>>mssv;
cout<<"nhap diem toan:";
	cin>>diemtoan;
cout<<"nhap diem ly  :";
	cin>>diemly;
cout<<"nhap diem hoa :";
	cin>>diemhoa;
	dtb=(diemtoan+diemly+diemhoa)/3.0;

}
void sinhvien::xuat()

{   cout<<setw(15)<<ten;
    cout<<setw(15)<<mssv<<setw(12)<<diemtoan;
    cout<<setw(12)<<diemly<<setw(12)<<diemhoa;
    cout<<setw(12)<<dtb;
    cout<<setw(12)<<hebon(dtb)<<endl;
	
}
string sinhvien::hebon(float dtb)
{
	if(dtb>=9&&dtb<10)      return "A";
	if((dtb>=8.5&&dtb<9))   return "B+";
    if((dtb>=7.5&&dtb<8.5)) return "B";
	if((dtb>=6.5&&dtb<7.5)) return "C+";
	if((dtb>=5.5&&dtb<6.5)) return "C";
	if((dtb>=4.5&&dtb<5.5)) return "D+";
	if((dtb>=3.5&&dtb<4.5)) return "D";
	if(dtb<3.5)             return "F";
		
}
int main()
{       cout<<setiosflags(ios::showpoint)<<setprecision(2);
        sinhvien sv[20];
        int n;
		cout<<"nhap so sinh vien:";
		cin>>n;
		for(int i=0;i<n-1;i++)
		for(int j=i+1;j<n;j++)
		if(sv[i].ten>sv[j].ten))
		{
		sinhvien str =sv[i];
		sv[i]=sv[j];
		sv[j]=str;	
		}
		for(int i=0;i<n;i++)
	    { 
		sv[i].nhap();
	    }
	    cout<<"----------------------------------------------------------------------------------------------------------------------"<<endl;
			    cout<<"      HOTEN          MA SV       DIEMTOAN        DIEMLY      Diemhoa     DIEMTB          DIEM HE 4 CHU  "<<endl;
		for(int i=0;i<n;i++)
		{ 
	    sv[i].xuat();
		cout<<endl;}
	
return 0;
}